It is known that HDL relates to removal of cholesterol accumulated in cells because it receives cholesterol from various tissues including walls of blood vessels with arterial sclerosis, so that HDL is useful for estimating the risk for various arterial sclerosises including coronary artery sclerosis, and that its blood level is an indicator for the risk of onset of arterial sclerosis.
Methods for measuring cholesterol in HDL include a method in which HDL is separated from other lipoproteins by ultracentrifugation and then the HDL is measured; and a method in which the cholesterol in HDL is separated by electrophoresis, then the lipid is stained, and the intensity of the generated color is measured. However, these methods are complex or a number of samples cannot be assayed, so that they are not commonly used.
The method for measuring the cholesterol in HDL, which is generally used in the field of clinical test is the method in which a precipitating agent is added to the sample so as to coagulate the lipoproteins other than HDL, removing the coagulated lipoproteins by centrifugation, and the cholesterol in the resulting supernatant containing HDL alone is measured. Although this method is simpler than the ultracentrifugation method and the electrophoresis method, it is not satisfactorily simple because it comprises addition of the precipitating agent and subsequent separation, and a comparative large amount of sample is needed.
On the other hand, methods in which the cholesterol in HDL is separately quantified by using enzymes have been proposed. For example, a method is known, which comprises the steps of preliminarily coagulating the lipoproteins other than HDL by an antibody and polyanion, enzymatically reacting the cholesterol in HDL alone, inactivating the enzyme and simultaneously re-dissolving the coagulated mass, and measuring the absorbance of the resulting solution (Japanese Laid-open Patent Application (Kokai) No. 6-242110). However, this method has a problem in that it is necessary to add reagents at least three times, so that this method can be practiced only by the limited analyzing apparatuses. Therefore, this method is not widely used. Other methods include a method in which an enzyme reaction is carried out in the presence of a bile salt or a nonionic surfactant (Japanese Laid-open Patent Application (Kokai) No. 63-126498); a more recently developed method in which the cholesterol in HDL is specifically trapped by chemically modified cholesterol esterase and/or cholesterol oxidase in the presence of a clathrate compound such as cyclodextrin (Japanese Laid-open Patent Application (Kokai) No. 7-301636); and a method in which the lipoproteins other than HDL are made into aggregates or complexes and then the cholesterol in HDL is trapped by an enzyme reaction (Japanese Laid-open Patent Application (Kokai) Nos. 8-131197 and 8-201393). However, with these methods, the results for certain samples are different from the results by the precipitation method, so that their specificities are problematic.
The present applicant previously developed a method for quantifying HDL cholesterol which does not necessitate a fractionating operation (International Publication No. WO98/26090), and the reagent therefor is now being generally used in the actual clinical tests. In this method, cholesterol in lipoproteins other than HDL in a sample is erased (the term “erase” herein means to decompose ester type cholesterol and free cholesterol, and to make the decomposed products undetectable in the second step), and HDL cholesterol is specifically quantified in the second step.
However, this method has a problem in that the measured amount of HDL is larger than the actual amount of HDL for abnormal clinical samples such as disorder of lipid metabolism and lipoprotein abnormality. Abnormal samples often indicate abnormal triglyceride (TG) values, bilirubin values and the like in biochemical tests, so that overcoming the above-mentioned problem will increase the usefulness of the measuring method, and so the solution of the problem is demanded.